Lower back and neck pain is oftentimes attributed to the rupture or degeneration of intervertebral discs due to degenerative disk disease, spondylolisthesis, deformative disorders, trauma, tumors, and the like. This pain typically results from the compression of spinal nerve roots by damaged discs between the vertebra, the collapse of the disc, or the resulting adverse effects of bearing the patient's body weight through a damaged unstable vertebral joint. To remedy this, spinal implants have been inserted between the vertebral bodies to restore the joint to its previous height and stabilize the motion at that spinal segment.
Surgical treatments to restore the vertebral height typically involve excision of the ruptured soft disc between the vertebrae, often with subsequent insertion of a spinal implant or interbody fusion device to fuse and stabilize the segment.
Spinal implants or interbody fusion devices have been used to fuse adjacent vertebral bodies since the 1960's. U.S. Pat. No. 6,261,586 to McKay and U.S. Pat. No. 6,123,731 to Boyce, et al. disclose spinal implant devices that are comprised of allograft materials. One major drawback associated with allograft devices is the risk of disease transmission. Further, since companies that provide allograft implants obtain their supply from donor tissue banks, there tend to be limitations on supply. Current synthetic devices, which are predominantly comprised of metals such as titanium, also present drawbacks. For instance, the appearance of metal spinal implants on x-ray tends to have an artificial fuzziness that makes assessment of fusion, which is one of the clinical criteria of a successful interbody fusion device, very difficult. Moreover, synthetic materials of this type tend to have mechanical properties that are unevenly matched to bone.
U.S. Pat. Nos. 5,681,872 and 5,914,356 to Erbe teach bioactive load bearing bone bonding compositions having a modulus of elasticity between 5 GPa to 50 GPa and added components that impart radiopacity. Erbe further teaches that the moduli of these compositions are closer to those of natural bone (7 GPa to 20 GPa) than PMMA alone (3 GPa to 5 GPa) or metal (100 GPa to 200 GPa). Erbe does not provide guidance as to a radiopacity range optimal for implants.
U.S. Pat. No. 6,261,586 to McKay teaches a composition of natural selectively deactivated bone mineral, which has a modulus of elasticity similar to the surrounding bone, as well as an approximate radiopacity of the bones of the vertebrae. McKay also discloses that commonly used implant materials have stiffness values far in excess of bone. The stiffness of cortical bone is 17 GPa. For instance, the stiffness of titanium alloy is 114 GPa, and the stiffness of 316L stainless steel is 193 GPa. Yet, there has been no showing of a synthetic material with a stiffness equivalent to bone.
U.S. Pat. No. 6,039,762 to McKay teaches a reinforced bone graft substitute in the form of an interbody fusion spacer composed of a porous, biocompatible ceramic material having a compressive strength of only at least 7 MPa and most preferably of only at least 40 MPa, and having the radiopacity of natural bone. U.S. Pat. No. 6,123,731 to Boyce, et al. teaches an osteoimplant fabricated from a solid aggregate of bone-derived elements having a compression strength between 10 MPa to 200 MPa and an added component that has the possibility of imparting radiopacity. U.S. Pat. No. 5,415,546 to Cox, Sr. teaches a radiopaque dental composition containing from about 10% to 80% of a radiopaque material such as diatrizoate sodium, barium sulfate, iodine or barium material. However, there has been no disclosure of a material with both mechanical properties similar to bone and an equivalent radiopacity of bone.
U.S. Pat. No. 5,024,232 to Smid teaches radiopaque heavy metal polymer complexes that have radiopacities equivalent to that of aluminum or higher. Again there is no guidance as to providing a synthetic material with radiopacity equivalent to bone.
Accordingly, there is a need in the art for a synthetic spinal implant material that does not carry the risk of disease transmission as with allograft materials.
There is also a need for a synthetic spinal implant material with a radiopacity similar to bone. A radiopacity similar to bone would allow for visualization of the implant between the vertebrae to assess radiographic fusion without distortion.
Further, there is a need for implants with mechanical properties similar to that of bone that can share the physiologic, dynamic compressive loads rather than shield them.
Moreover, there is a need for implants that are comprised of a material that bonds directly to bone and is bioactive.